The subject matter disclosed herein relates to an active rotor damping system and, more particularly, to an active lead-lag damping system for articulated rotor systems.
Rotary-wing aircraft include rotor systems and rotor blade assemblies to generate lift and allow for controlled operation of the air-vehicle.
During rotorcraft operation, the rotor blades are influenced by aerodynamic and inertial forces. As a result, each blade will experience elastic deformation as well as rigid body motion as a consequence of the forces acting upon it, referred to herein as blade dynamics. As a result of the blade dynamics, rotor systems are typically susceptible to forms of aero-elastic and aero-mechanical instabilities.
One form of instability is ground resonance, which is a cumulative, system-level effect of the individual blade dynamics. The motion of the blades about their respective lead/lag axis couples with the natural frequency of the landing gear and fuselage. In this mode, there is a migration of the rotor system center of gravity away from the axis of rotation. If this migration continues in an increasing and divergent manner, the system is deemed to be “unstable.” Another form of instability includes air-resonance, where the cyclic motion in the rotor system couples with a structural natural frequency of the fuselage in forward flight and introduces undesirable vibration inputs to the airframe. Instability can have a significant, adverse impact on the operation of the air-vehicle, including the damage to rotor system and potentially loss of the air-vehicle.
Stability is heavily influenced by damping; where damping is a force that acts upon an object that acts in the opposing direction of motion and is proportional to the velocity of the object. In the context of a helicopter blade, the profile of the blade is capable of producing aerodynamic damping. However, in the direction of rotation, also referred to as the edgewise direction, the blade has a very limited profile. This reduced profile is advantageous from an aerodynamic perspective as it minimizes drag induced aerodynamic forces. But conversely, it severely degrades the aerodynamic damping potential in this plane of motion.
Thus, a typical feature of an articulated rotor system is the lead/lag damper, which is usually mounted between the hub and the rotor blade within the lead-lag reference-plane. These devices serve to augment the aerodynamic and structural damping of the rotor blade in the lead/lag direction. As the blade lag-motion (and corresponding velocity) increases, the dampers produce an increasing resistive force in order to minimize cyclic forces generated within the rotor system and ensure avoidance with the other inherent natural frequencies of the air-vehicle so as to ensure stability of the rotor system. Typically, the lead/lag dampers are sized for a variety of considerations including the need to stabilize ground resonance of the rotary-wing aircraft.
Although effective, current dampers make use of hydraulic restriction as the primary damping mechanism. They operate under high pressures and may require relatively comprehensive maintenance attention. Furthermore, as typical lead/lag damper technology is passive, the dampers subject the rotor systems to loading during conditions in which the additional force produced by the damper is not required in order to maintain rotor system stability. Thus, the overall structural envelope and weight of the rotor systems such that aircraft weight is concomitantly increased.